1. Field of the Invention
An embodiment of the present invention relate to a liquid crystal display substrate, and a liquid crystal display device including the liquid crystal display substrate.
2. Description of the Related Art
In regard to a thin display device such as a liquid crystal display device, there has been a demand for an enhancement of image quality, price reduction, and electric power saving. A color filter for the liquid crystal display device is required to have sufficient color purity, high contrast, flatness and the like for the purpose of display with high image quality.
Regarding an alignment mode of liquid crystals or a liquid crystal driving system that is applied to the liquid crystal display device with high image quality, VA (Vertical Alignment), HAN (Hybrid-aligned Nematic), TN (Twisted Nematic), OCB (Optically Compensated Bend), CPA (Continuous Pinwheel Alignment) and the like have been suggested. When such an alignment mode of liquid crystals and liquid crystal driving system are used, a display device with wide viewing angle and high-speed response is realized.
In the VA mode, liquid crystals are aligned in vertical to a surface of a substrate such as glass. This VA mode is effective in the realization of high viewing angle and high speed response. The HAN mode is effective in the realization of high viewing angle. In the liquid crystal display device such as the VA mode and HAN mode, high level of flatness (uniformity of film thickness or a reduction of surface asperities of a color filter surface) and an electrical characteristic such as dielectric constant are required for a color filter. In a liquid crystal display device with high image quality, due to a decrease in coloration upon diagonal viewing, a technology of making a liquid crystal cell thickness (thickness of the liquid crystal layer) small is important. In regard to the VA mode, the development of various improved modes such as MVA (Multi-Domain Vertical Alignment), PVA (Patterned Vertical Alignment), VAECB (Vertical Alignment Electrically Controlled Birefringence), VAHAN (Vertical Alignment Hybrid-Aligned Nematic) and VATN (Vertical Alignment Twisted Nematic) is underway. Furthermore, in a liquid crystal display device of a longitudinal electric field mode in which a driving voltage is applied in a thickness direction of the liquid crystals, such as the VA mode, it is desirable to realize higher speed response of liquid crystals, a wider viewing angle, and a higher transmittance. In a conventional liquid crystal display device, when a voltage is applied to liquid crystals that are initially vertical to the substrate surface, a direction in which the liquid crystals would tilt is not easily determined, and vertically aligned liquid crystals may become unstable upon an application of a liquid crystal driving voltage. In order to avoid that vertically aligned liquid crystals become unstable as such, in the MVA technology, plural slit-like convex areas are provided, and liquid crystal domains are formed between these slits so as to have plural directions of alignment. Thereby, in the MVA technology, a wide viewing angle is secured.
Patent Literature 1 (Japanese Patent No. 3957430) discloses a technology for forming a liquid crystal domain by using a first and a second alignment regulating structure (slit).
Patent Literature 2 (Jpn. Pat. Appln. KOKAI Publication No. 2008-181139) discloses a technology for forming four liquid crystal domains by using photo-alignment. In this Patent Literature 2, it is disclosed that in order to secure a wide viewing angle, multiple alignment treatments related to strict control of the tilt angle (89°) and plural alignment axes that respectively differ by 90° are needed at each of the domains.
Patent Literature 3 (Japanese Patent No. 2859093) and Patent Literature 4 (Japanese Patent No. 4364332) disclose a technology for controlling liquid crystals that are vertically aligned by an oblique electric field by a transparent conductive film (a transparent electrode, a display electrode or a third electrode) of a color filter substrate side and a first electrode and a second electrode of an array substrate side. In the Patent Literature 3, liquid crystals having negative dielectric constant anisotropy are used. In Patent Literature 4, liquid crystals having positive dielectric constant anisotropy are described. Meanwhile, Patent Literature 4 describes nothing about liquid crystals having negative dielectric constant anisotropy.
Usually, in a basic configuration of the liquid crystal display device of VA mode, TN mode or the like, the liquid crystals are sandwiched by a color filter substrate equipped with a common electrode, and an array substrate with plural pixel electrodes that drive liquid crystals (for example, transparent electrodes that are electrically connected to TFT elements and are formed in a comb-shaped pattern). In this configuration, a driving voltage is applied between the common electrode above the color filter and the pixel electrodes formed for the array substrate side, and liquid crystals are driven by the application of this driving voltage. As the transparent conductive film that is used as the pixel electrode or the common electrode of the color filter surface, a thin film of an electroconductive metal oxide such as ITO (indium tin oxide), IZO (indium zinc oxide) or IGZO (indium gallium zinc oxide) is usually used.
Patent Literature 5 (Jpn. Pat. Appln. KOKAI Publication No. 10-39128) discloses a color filter which includes blue pixels, green pixels, red pixels, a black matrix, and the like. In this Patent Literature 5, for example, while a black matrix is formed, a transparent conductive film is also formed above the color pixels, and an overcoat is further laminated thereon.
A technology of forming the cross-section of the black matrix into a trapezoidal shape is disclosed in the Patent Literature 3. In the Patent Literature 4 (for example, FIGS. 7 and 9), a technology of using plural stripe electrodes and liquid crystals having positive dielectric constant anisotropy is disclosed. In this Patent Literature 4, a color filter is formed above a transparent electrode (transparent conductive film).
Patent Literature 6 (Jpn. Pat. Appln. KOKAI Publication No. 5-26161) discloses a technology of forming a color filter above a transparent conductive film.
As explained above, in a vertically alignment liquid crystal display device employing the MVA technology, domains of liquid crystals are formed by alignment regulating structures called slits, in order to secure a wide viewing angle. When the liquid crystals have positive dielectric constant anisotropy, the liquid crystals located between two slits made of a resin that are formed above the color filter or the like tilt upon the application of a driving voltage, for example, in a direction perpendicular to these slits as viewed in a planar view, and the liquid crystals, except for the liquid crystals at a center between the two slits, move to line up horizontally above the substrate surface. However, the liquid crystals at the center between the two slits do not have the direction of tilt definitively determined despite the voltage application, and may be in a spray alignment or bend alignment state. Such alignment disorder of liquid crystals may cause a rough texture in the liquid crystal display or display unevenness. In the case of the MVA mode, in addition to the problems described above, it is difficult to finely control the amount of tilt of the liquid crystals by means of the driving voltage, and it is difficult to achieve a half-tone display satisfactorily. Particularly, the linearity in the relationship between the driving voltage and the display (response time) is low, and it is difficult to achieve a half-tone display satisfactorily with a low driving voltage.
In order to solve such problems, as disclosed in Patent Literature 3 or Patent Literature 4, a technique of controlling the liquid crystal alignment with an oblique electric field by using first, second and third electrodes is effective. The direction of tilt of the liquid crystals can be set by the oblique electric field. Furthermore, it becomes easy to control the amount of tilt of the liquid crystals by the oblique electric field, and a half-tone display can be achieved satisfactorily.
However, there are occasions in which even if these technologies are used, the measures for disclination of liquid crystals are unsatisfactory. Disclination means that regions with different light transmittances occur within a pixel due to unintended alignment disorder or non-alignment of liquid crystals. Here, the pixel is the smallest unit of liquid crystal display.
In the Patent Literature 3, due to the fixing of disclination at the center of the pixel, an alignment control window where there is no transparent conductive film at the center of the pixel of a counter electrode (third electrode) is provided. However, Patent Literature 3 does not disclose any remedial measures for disclination in the periphery of the pixel. Furthermore, in the Patent Literature 3, fixing of disclination at the center of the pixel is enabled, but no investigation was conducted regarding the measures to minimize disclination. Furthermore, Patent Literature 3 does not describe any technology for improving the responsiveness of liquid crystals.
Patent Literature 2 discloses that strict control of the tilt angle of liquid crystals to 89 degrees and four times of alignment treatments are required in order to secure a wide viewing angle.
In Patent Literature 4, when a dielectric layer is laminated above a transparent conductive film (transparent electrode), the effect of the oblique electric field is enhanced, and preferable results are obtained. However, as shown in FIG. 7 of Patent Literature 4, there are occasions in which vertically aligned liquid crystals remain at the center of the pixel and the edges of the pixel even after voltage application, and the transmittance or numerical aperture decreases. Also, in regard to Patent Literature 4, in the case of using liquid crystals having positive dielectric constant anisotropy, it is difficult to increase the transmittance due to disclination at the center of the pixel. Meanwhile, in Patent Literature 4, no investigation was conducted on liquid crystals having negative dielectric constant anisotropy.